Why am I getting this yellow warning message...

the user function does have a domain - it's fn!

If you write it as fn!user() that message will disappear.

Some context: these are the available domains:

...and it's now been deemed best practice to prefix Appian constructs with their respective domain.

(just realized fn! doesn't appear in the above list, which seems to me an oversight! - I'll raise this with Appian Engineering)

Stewart Burchell said:

and it's now been deemed best practice to prefix Appian constructs with their respective domain

Since when?  If this were the case, I'd expect that using the auto-fill rule picker in an expression/interface would populate the "fn!" domain like the "a!" domain is autopopulated - and at least as of 21.2, it isn't.

Also, FWIW, i'm unable to reproduce the warning message as seen in the initial screenshots here - I wonder if there might be some other issue at play?  Or do some people have ultra-early access to 21.3?

Prefixing Appian constructs has always been informally a best practice (if that's not an oxymoron! - I always insisted on it when peer reviewing code) and if you use the function picker (fx) in an Expression Editor that does append the relevant domain prefix (in 21.1)

If it's missing in the context you call out then I'd assert that needs to be amended.

...I also should have said that what you're seeing is not an "error" but a "recommendation". Appian has put investment into building implementation recommendations into the platform to help developers delivery quality applications. You can see these kinds of recommendations appearing in the Designer tools (like the Expression editor) and also in the 'Health Dashboard' tab of the 'Monitoring;' view within an Application.

function prefixes are not always required. The expression editor warning will typically flag a function when there is a local variable or rule input with the same name. That's why 'user' is flagged but 'loggedInUser' is not